HF Shortwave Radio Propagation Information - A Index and K Index

A Index and K Index

The Earth's magnetic field is continuously monitored by a network of magnetometers. These
readings are converted into the A and K index values.

The K index is computed once every three hours (eight times a day) and the values can range from 0
to 9, with 0 being inactive, and 9 representing an extreme severe storm condition. The values
are quadi-logarithmic.

K = 0 Inactive

K = 1 Very quiet

K = 2 Quiet

K = 3 Unsettled

K = 4 Active

K = 5 Minor storm

K = 6 Major storm

K = 7 Severe storm

K = 8 Very severe storm

K = 9 Extremely severe storm

The A index is linear, and is computed from the eight previous K index values. It ranges from
0 (quiet) to 400 (severe storm).

A = 0 - 7 Quiet

A = 8 - 15 Unsettled

A= 8 - 15 Unsettled

A = 16 - 29 Active

A = 30 - 49 Minor storm

A = 50 - 99 Major storm

A = 100 - 400 Severe storm

Generally, propagation conditions are best when the A index is 15 or lower, and the K index is
3 or lower.

Besides causing auroral activity, high geomagnetic field conditions can affect the electrons in
the ionosphere, reducing the maximum usable frequency (MUF).